The focus of this thesis is to explore interactivity within musical composition. I start by examining the evolution of the relationship between artist, medium, and audience during the 20th century regarding how it distanced itself from a primarily hierarchical construct towards a more interdependent and flexible structure. The once established model where the composer's creative voice was the only one echoed in one's work has thereafter been challenged. Gradually, new compositional techniques appeared that allowed performers to influence the musical outcome of a composer's work (e.g. compositional techniques such as indeterminism, alternative notation system such as graphical scores, etc.). Towards the end of the 20th century, technological advances in the field of human-computer interaction (HCI) accelerated this trend by providing artists with the ability to construct customised musical interactive interfaces. Such interfaces have the potential to facilitate the involvement of audience members and extend the role of performers so they can leave a temporal mark on the final artistic work. In the course of my research thesis, I experiment with the designing of such musical interactive interfaces. I focus on the creation of solo projects, aiming at assisting studio based composition and enhancing musical performance, as well as collaborative cross-disciplinary projects (involving choreography, theatre, etc.) where the performers' and the audience's participation influence the musical outcome. In my commentary, I describe with a practical perspective the reasoning behind their physical, as well as conceptual attributes on a per case study basis.